The present invention relates in general to fabrication of semiconductor devices and in particular to an electron beam exposure system for writing a semiconductor pattern on a semiconductor substrate by an electron beam.
In the submicron patterning of semiconductor devices, the electron beam exposure system is suited. The electron beam exposure system uses a finely focused electron beam for writing a semiconductor pattern on a semiconductor substrate and can achieve the resolution of less than 1 .mu.m without difficulty. On the other hand, the conventional electron beam exposure system has suffered from the problem of relatively low throughput because of the basic constraint of the system in that the semiconductor pattern is written in one stroke of the focused electron beam.
In order to improve the problem of low throughput, a technique of so-called block exposure is proposed. According to this procedure, the electron beam is shaped into one of fundamental patterns of semiconductor devices and a desired semiconductor pattern is written on the substrate as a consecutive repetition of "shots" of selected fundamental patterns. This block exposure technique is particularly suited for the fabrication of semiconductor devices such as memories in which a repetition of fundamental patterns is included.
In the block exposure process, the shaping of the electron beam is achieved by deflecting the electron beam to pass through apertures that are provided on a stencil mask. By addressing various apertures on the mask, one can shape the cross section of the electron beam according to the desired fundamental pattern. Conventionally, an electrostatic deflector is used for such a high speed deflection of the electron beam because of the extremely fast response. Such an electrostatic deflector is used also in various places of the electron beam exposure system.
FIG. 1 shows the construction of a conventional electron beam exposure system that uses the technique of block exposure. Referring to the drawing, the electron beam exposure system is generally formed from an electron optical system 100 for producing and focusing an electron beam and a control system 200 for controlling the optical system 100.
The electron optical system 100 includes an electron gun 104 as a source of the electron beam. The electron gun 104 includes a cathode electrode 101, a grid electrode 102 and an anode electrode 103, and produces the electron beam generally in the direction of a predetermined optical axis 0 in the form of divergent beam.
The electron beam thus produced by the electron gun 104 is passed through a shaping aperture 105a formed in an aperture plate 105. The aperture plate 105 is provided such that the aperture 105a is in alignment with the optical axis O and shapes the incident electron beam to have a rectangular cross section.
The electron beam thus shaped is received by an electron lens 107a that has a focal point coincident to the aperture 105a. Thereby, the incident electron beam is converted to a parallel beam and enters into an electron lens 107b that focuses the electron beam on a block mask 110. It should be noted that the lens 107b projects the image of the rectangular aperture 105a on the block mask 110. As shown in FIG. 2, the block mask 110 carries a number of fundamental patterns 1a, 1b, lc, . . . of the semiconductor device pattern to be written on the substrate in the form of apertures, and shapes the electron beam according to the shape of the aperture through which the electron beam has passed.
In order to deflect the electron beam passed through the electron lens 107b and address the desired aperture, electrostatic deflectors 111, 112, 113 and 114 are provided, wherein the electrostatic deflector 111 deflects the electron beam away from the optical axis O in response to a control signal SM1. The electrostatic deflector 112 in turn deflects back the electron beam generally in parallel to the optical axis O in response to a control signal SM2. After passing through the block mask 110, the electrostatic deflector 113 deflects the electron beam toward the optical axis O in response to a control signal SM3, and the electrostatic deflector 114 deflects the electron beam such that the electron beam travels coincident to the optical axis O in response to a control signal SM4. Further, the block mask 110 itself is provided movable in the direction perpendicular to the optical axis O for enabling the addressing of the apertures on the entire surface of the block mask 110 by the electron beam.
The electron beam thus passed through the block mask 110 is then focused at a point fl that is located on the optical axis O after passing through electron lenses 108 and 116. There, the image of the addressed aperture on the block mask 110 is demagnified at the point fl. The electron beam thus focused is then passed through a blanking aperture 117a formed in a blanking plate 117 and further focused on the surface of a substrate 123 that is held on a movable stage 126, after passing through electron lenses 119 and 122 that form another demagnifying optical system. There, the electron lens 122 serves for an objective lens and includes various coils such as correction coils 120 and 121 for focusing compensation and stig compensation as well as deflection coils 124 and 125 for moving the focused electron beam over the surface of the substrate 123.
It should be noted that the foregoing blanking aperture 117a is provided coincident to the optical axis O for establishing an alignment of the electron beam therewith. For this purpose, various adjustment coils 127-130 are provided. Thus, at the beginning of the exposure, the electron beam is turned on and the arrival of the electron beam at the stage 126 is detected while controlling the adjustment coils 127-130. During this procedure, the mask 110 may be removed from the optical path O for free passage of the electron beam. Alternatively, a large aperture formed in the mask 110 for passing the electron beam freely may be used.
In FIG. 1, it should be noted that the illustrated state represents the operational state of the electron beam exposure system wherein the electron beam is focused at the surface of the substrate 123. In this state, the focusing point fl of the optical system formed of the lenses 108 and 116 is located above the blanking aperture plate 117 for achieving the desired demagnification.
In order to control the exposure operation, the electron beam exposure system of FIG. 1 includes the control system 200, wherein the control system 200 includes memory devices such as a magnetic tape device 201 and magnetic disk devices 202, 203 that are provided to store various data of the device pattern of the semiconductor device to be written. In the illustrated example, the magnetic tape device 201 is used for storing various design parameters, the magnetic disk device 202 is used for storing the exposure pattern data, and the magnetic disk device 203 is used for storing the pattern of the apertures on the block mask 110.
The data stored in the memory devices is read out by a CPU 204 and transferred to an interface device 205 after data decompression. There, the data for specifying the pattern on the block mask 110 is extracted and stored in a data memory 206. The data stored in the data memory 206 is then transferred to a first control unit 207 that produces the foregoing control signals SM1-SM4 and supplies the same to the electrostatic deflectors 111-114. Further, the control unit 207 produces and supplies a control signal to a mask moving mechanism 209 that moves the block mask 110 in the direction transverse to the optical path O. In response to the deflection of the optical the electrostatic deflectors 111-114 and further in response to the lateral movement of the block mask 110, one can address the desired aperture on the mask 110 by the electron beam.
The first control unit 207 further supplies a control signal to a blanking control unit 210 that in turn produces a blanking signal for shutting off the electron beam. This blanking signal is then converted to an analog signal SB in a D/A converter 211 and the analog signal SB is supplied to an electrostatic deflector 115 that causes a deflection of the electron beam away from the optical axis O. In response to this, the electron beam misses the blanking aperture 117a and disappears from the surface of the substrate 123. Further, the control unit 207 produces a pattern correction data H.sub.ADJ and supplies the same to a D/A converter 208. The D/A converter 208 in turn produces a control signal S.sub.ADJ and supplies the same to an electrostatic deflector 106 that is provided between the electron lens 107a and the electron lens 107b. Thereby, one can modify the shape of the electron beam that have passed through the addressed aperture in the mask 110. This function is used when the desired shape of the electron beam is different from the shape given by the apertures on the block mask 110.
The interface device 205 further extracts and supplies the data for controlling the movement of the electron beam on the surface of the substrate 123 to a second control unit 212. In response thereto, the control unit 212 produces a control signal for controlling the deflection of the electron beam on the surface of the substrate 123 and supplies the same to a wafer deflection control unit 215 that in turn produces and supplies deflection control signals to D/A converters 216 and 217. The D/A converters 216 and 217 in turn produce drive signals SW1 and SW2 for driving the deflectors respectively and supply the same to the deflectors 124 and 125 for causing the deflection of the electron beam. Thereby, the position of the stage 126 is detected by a laser interferometer 214 and the wafer deflection control unit 215 modifies the output deflection control signals and hence the drive signals SW1 and SW2 according to the result of measurement of the stage position by the laser interferometer. Further, the second control unit 212 produces a control signal that causes a lateral movement of the stage 126.
FIG. 2(A) shows a conventional electrostatic deflector that is used in the electron beam exposure system of FIG. 1.
Referring to FIG. 2(A), the electrostatic deflector includes an insulating sleeve 10 of a ceramic material, and there are provided a number of elongated electrodes 11 along the inner surface of the sleeve 10 to extend in the longitudinal direction of the sleeve. In the cross sectional view of FIG. 2(A), the electrodes 11 are arranged symmetrically about a central axis of the sleeve. In operation, a positive voltage +V and a negative voltage -V are applied across the opposing electrodes to form an electric field inside the sleeve as indicated by the broken lines. Thereby, the electron beam that travels along the central axis of the sleeve 10 is deflected by the Lorentz force as is well know. Further, in order to realize a uniform distribution of the electric field inside the sleeve 10, the rest of the electrodes 11 are also applied with a voltage V.sub.i that changes with an angle .theta. according to the relationship V.sub.i =V.multidot.sin .theta..
In the electrostatic deflector of FIG. 2(A), there exists a problem in that the electrons tend to be accumulated on the inner surface of the ceramic sleeve 10 when used for deflecting the electron beam. It should be noted that there exit various scattered electrons inside the evacuated column that accommodates the electron optical system of the electron beam exposure system, and the accumulation of such scattered electrons on the sleeve of the electrostatic deflector cannot be avoided. Such an accumulation of the electrons causes a charge-up of the electrostatic deflector, while such a charge-up of the electrostatic deflector causes in turn an unwanted deflection or deformation of the electron beam. This problem of charge-up is particularly serious when writing a semiconductor pattern with the submicron resolution. For example, the exposed pattern may be deformed or blurred.
The conventional electrostatic deflector of FIG. 2(A) has another problem in that the distribution of the potential inside the sleeve 10 becomes inevitably distorted particularly in the vicinity of the electrodes. It should be noted that there is no potential inside a conductor body. Thus, there occurs a problem in that the electric field formed inside the sleeve 10 becomes distorted significantly in the vicinity of the electrodes. In order to achieve a uniform electric field inside the sleeve 10, it is preferable to form the size of the electrode as small as possible, particularly in terms of the lateral width. On the other hand, the formation of such fine, strip-like electrodes on the inner surface of the sleeve 10 with a precision sufficient for the submicron patterning is extremely difficult.
FIG. 2(B) shows another example of the conventional electrostatic deflector wherein the problem of charge-up is eliminated.
Referring to FIG. 2(B), the electrostatic deflector includes an insulating sleeve 20 similar to the ceramic sleeve 10, wherein there is provided a conductor 20a such that the conductor 20a covers the inner surface of the sleeve 20. As indicated in FIG. 2(B), the conductor 20a is connected to the ground. Further, there are provided a plurality of electrodes 21 arranged generally symmetrical about the axis of the sleeve 10 along the inner surface of the conductor 20a but with a separation therefrom. There, the electrodes 21 extend in the axial direction of the sleeve 20. In the illustration of FIG. 2(B), the electrodes extend perpendicular to the sheet of the drawing.
In the electrostatic deflector of FIG. 2(B), the problem of the charge-up is successfully eliminated. On the other hand, the electrostatic deflector of FIG. 2(B) has a problem in that the fabrication thereof is extremely difficult, as the electrodes 21 have to be held exactly with a predetermined relationship with each other and further with respect to the conductor 20a. It should be noted that the inner diameter of the sleeve 20 is in the order of several millimeters. Further, the problem of the non-uniform distribution of the potential and hence the electric field is not solved in the device of FIG. 2(B).
FIG. 3 shows another electrostatic deflector that also uses an insulating sleeve 30.
Referring to FIG. 3, the insulating sleeve 30 is formed with a number of grooves 31 on the inner surface thereof to extend in the axial direction of the sleeve. There, the grooves 31 have a T-shaped cross section and are disposed symmetrically about the central axis of the sleeve. Further, the inner surface of the sleeve 30 as well as the surface of the groove 31 are covered by a conductor film 32 except for a part 32a such that the conductor film 32 is divided into a number of separate electrodes 33. In the electrostatic deflector of FIG. 3, the entire inner surface of the sleeve 30 is covered by the conductor film 32, except for the part 32a, and the problem of the charge-up is eliminated almost completely. On the other hand, the electrostatic deflector still has the problem of non-uniform distribution of the potential because of the relatively large lateral extension of the electrode 33.
Further, the electrostatic deflector of FIG. 3 has another more practical problem in that the fabrication of the device is extremely difficult. Generally, the ceramic sleeve that has such a complex cross section is formed by the injection molding process or extrusion process. In such a forming process used commonly in the field of ceramic materials, it is generally impossible to obtain a smooth surface inside the grooves 31. It should be noted that the inner surface of the sleeve 30 is made smooth by polishing. On the other hand, polishing of the inner surface of the grooves 31 is impossible. When the conductor film 32 is formed on such a rough surface of the groove 31 by a plating process or the like, there is a substantial risk that small pinholes are formed on the conductor film 32 and such pinholes tend to cause the charge-up. As already noted, such a charge-up may cause an unwanted deformation or blur of the pattern that is written on the surface of the substrate.